Thermionic tube circuit



Oct. 12, 1937.

D. G. MccAA THERMIONIC TUBE CIRCUIT Filed Feb. 1v1, 1935 2 Sheets-Sheet 2 K una. .SSM .2mg Erw l Wmbh @200.31

. H mgmt param] oi-Fici-z David GQiuccn, ic ScarsdalcgfN.-Y .,'astrnlteo` 'iiiuiii'cation February ii, insa-sensi Nc. m14

' umleiten-zii" My'. invention relates generally te thermionic minal of- M 4is conn j to the resistance I5 tube circuits. vMore specically I lia've-V devised which is, by preference, n-inductive. The reimprovements in thermionic* Vtube circuits in maining terminal of the resistance I is joined which characteristic grid voltage-plate current to plate 6v and also connected to output condenser 5 curvesmay be obtained which 4have a sloping I6. -The output terminals are'i'l and Il.' f '5 po tion which abruptly changes to horizontal. A- suitable'by-pass condenser l8` isconnected ne of the objects of my invention is to profrom the positive terminal of i4 to the ground vide -a vacuum tube circuit arrangement .in which connection I2. A potentiometer!!! is connected the grid voltageplate current characteristich across i3 and the movable terminal of the' po- 10 changes abruptly from a slope to aV-horimntal itentiometer 2l is connected to screen grid 5. A 10 line as the grid `electrodeismade less negative vby-pass condenser 2| iii connected from screen or more positive. Another object is the anim. grid to cathode. It will be seen that the screen ment of a detector or lrectier circuit'in lwhich grid .may be given' a suitable potential with rethe positive 'cycles of alternating currents, imspect to the cathode from zero topositiveby pressed on the input circuit.- llvbe completely means ofthe potentiometer 20. l5-

eliminated inthe output circui while thenega- It should be observed that the normal chartive potentials of the signal currents will be acteristic curve of grid voltage plotted against faithfully4 reproduced without'distortion. platecurrent is a' smooth .uniform curve. The

A furtherobject of-my-in'iprovement is'ioundl curve' shown in Figure II represents a normal in the sharp( cut-'off characteristic'which maybe tube characteristic. 1 Although the curve of Flgm had by practicing my invention. Another object ure 1I may be straightened somewhat by the use lies in avacuum tube circuit in rwhich the output zof -fan 'external plateI resistance, the upper and anode current is sharply'limited for an increas lower portionswill gradually curve. ingnegative input grid'voltage as well as anin- By the followingtahle those skilled creasing Positive inplltlrid voltage. Further obin the art will recognize that I employ voltages. 25

jects Willbe mere apparent from the accompnyand currents which are many times lower than ing drawings and specication.4 are recommended by 'the tube manufacturers.

. Figure I is .a circuit embodying invention, Bymeans o f these unsuallylow voltages, suitable in a single screen gridtube. biasing, and relatively high external resistance,

` 3c Figure II is the srid' Vvoltiise-plate mirent' ,r have arrived at operating ciicracteristicsen- 30 ehl'eerltle elll'veefftliewnvetionel tirely diiferent from those previouslyemployed. Figure III illlliete 'ifemily 0f Elid 'voltage- $31- way of example, the following table shows Plate eurent ehel'eeterietie' eullveeiei the,v eilfelllt operating constants for two types of tubes under ofFlgureI. .given conditions:

FigureIV is; il.I graphic illustrationof afchar- 35 acteristic detection action'of'the circuit shown" 1 snode 0M inlllurel- .f Y n vci Figure Vfshowsmy invention. appli`ed to'two Tyi'ietubc Egg' ma' my 5gg 'M etna A tubesforeifecting ad'ouble cut-oi or gate charv mmf Y lm' ,ppm

macteristic.; mf M 40 FigureVIisa graphic illustration of-an operatim; characteristic crthc circuitof mgm-en xg; {i} 2 2&2

In Figure 1.1! vrepresents a screen grid therv 'm a1 aso 22 o ai cs micnictub'c. in-wiiicii z :c4-thc ccntirci-jgridnis 1 gg" 2g 2g :iig

'the cathode,lthe heater, B the screen, and! the a7 1.o c o -ao 5 anode. The input voltag'eis applied between# 17 M5 9* M and' 8. LA suitable bias, either positive ornegative, may be appliediby mea'nsof potentiometer figure vIII illustrates a family 0f curves which I which is connected lacross-batteryll. Terminal may be obtained from the circuit of Figure I, and

.n l 4is connected by bij-pass.condenser` Il tothe the operating constants shown in the above table.

cathode 3 -which is grounded at i2. B batteries It'will be apparent' that the plate current is subf Il and M are connected in series. The-negative stantially unchanged as the grid potential is vaterminal of I3 `is connected to` the cathode 3 and fled from the extreme positive Voltage t0 lower ground I2. The positive rterminal fof Il connects values until a critical angle is reached. I have tothe negative terminal of M. j The positive ter-- c ii'llcl this angle A. It will also appear that angle 55 A may be made to occur at positive, zero or negative grid voltages. The dotted line IG shows the grid current which may occur with the various curves of the family. The most preferable condition is represented by the curves marked P or Q because at these angles the grid current is zero. 'I'he condition of zero grid currentl is desired because no load is placed on the circuit to which my device may be connected. I also prefer to use a characteristic curve which has a slope with straight line portion.

The angle A mayv be positioned or located on any part of the characteristic curve by adjusting the B battery voltage, the value of the external anode resistance, and the screen grid voltage for any given tube. sistance, the lower the B battery voltage, and the higher the screen grid voltage the more the angle A is shifted from positive to negative grid voltages; i. e., from M toward Q of Figure III. Likewise for increasingly high positive screen grid potentials, with a given B battery voltage and external anode resistance, the more negative or less positive the grid voltage at which angle A occurs.

In Figure III for each of the curves, a horizontal portion is met `by an angular portion which finally bends and approaches zero.. The characteristic may be changed from a steep to a gradual slope by making the screen grid increasingly positive with respect to the cathode. Since the useful portion of the characteristic curve is the slope, it should be understood that the less the slope the greater may be the applied grid voltage Avariations and vice versa.

I prefer the screen grid type of tube. How-l ever, the invention may be applied to other types of tubes. While the RCA 227 tube will give results similar to the characteristic illustrated in Figure III, it will be found that at radio frequencies the interelectrode tube capacities will interfere with complete realization of myI invention. 'I'his objection may be overcome by neutralizing the grid plate capacity by any of the well known methods. Even in the case of a screen grid tube, it may be desirable to neutralize the small interelectrode capacities present.

Tubes With uni-potential cathodes have characteristics which are especially suited to my circuits owing to the sharp angle obtainable, but it should be understood that I am not limiting my invention to any particular type of tube. I have found a wide range of tubes, potentials and resistances will accomplish the sharp angular cutoff shown in Figure III.

I do 'not with to limit my disclosure to any particular theory of operation in setting forth the following: The cathode is heatedin the normal manner. Electrons are emitted from the heated cathode. The number of emitted electrons reaching the anode is relatively low because the potential of the anode with respect to cathode is in the order of zero or slightly positive. The electrons passing from the filament form aspace charge which further restricts or limits the number of electrons which can reach the plate. Since the number of electrons which can reach the plate is limited by the space charge, the' grid electrode has no effect upon the plate current, until the grid' reaches a very critical potential at angle A. In actual practice the location of angle A is so precise and sharp, that it is diicult to determine the exact grid voltage at which it occurs. For increasingly less negative Or more The higher the external repositive grid voltages beginning at angle A, the plate current remains absolutely constant.

I shall now describe one application of my invention. In Figure IV, I show how the characteristics of the circuit of Figure I may be applied to the detection of radio frequency currents. The terminals 'l and 8 are connected to a source of radio frequency current. The connection may include a tuned circuit in which capacity and inductance are used. A signal reproducing instrument may be connected to the output terminals l'l and i8. 'I'he potentiometer 9 is adjusted until the grid is slightly negative as shown in Figure IV as represented by the dotted line X. It will be observed that X coincides with the critical angle A. The radio frequency currents are represented by the solid lines between Y and Z. Since the impulses between X and Y are equivalent to a relatively less negative grid, it will be seen that they do not alter the plate current. On the other hand the XZ impulses make the grid more negative and appear, without distortion, as changes in plate current. The above arrangement is a substantially perfect detector and avoids the distortional effects which take place in the conventional triode and pentode detector circuits.

While the arrangement thus far described is useful as a detector and limiter tube, it has limitations Which interfere with its use in what I have termed a gate. There have been numerous proposals in the art to employ thermionic tubes as limiters. One conventional use has depended upon producing saturation effects in the anode current by reducing the cathode emission. Another use depended upon loading the tube by excessive grid currents. 'I'hese limiters have been characterized by the relatively high voltages 'required before the limiting action became effective and the distortion due to the bending of the characteristic curves. Although 4these devices may have had a useful limiting action for telegraphic signals, they were relatively useless where telephonie currents were involved. This failure is due to the requirement of excessive input voltages and distortion. Many of the prior' art limiters were extremely ineicient because` they were loss devices.

In contrast to these limiters, I propose a gate which will cut off at relatively low voltages and pass radio frequency currents, up to the cut oi f characteristic angle A of Figure II but I can i also obtain a second or complementary angle B (see Figure VI). Furthermore, by suitable adjustments I can determine the size of the gate, or the limitsof voltage impulses it will pass.

In Figure V, 3| represents a screen grid thermionic tube, in which 32 is the control grid, 33 is the cathode, 34 thc.heater, 35 the screen, and 36 the anode. The input voltage isapplied between 31 and 38. A suitable grid bias, preferably negative, may be applied by means of potentiometer 39 which is connected across battery 40 whose positive terminal joins 33. Terminal 38 is connected by by-pass condenser 4I to the Cathode 33. The cathode is grounded at 42. A,

, A by-pass condenser 41 is connected between the positive terminal 01'43 and ground 42 Another .by-pass condenser 48 may be used between Il and 35.

Thus far the description of Figure V is substantially the same as Figure I. A second screen grid tube 5I, with circuits similar to thosealready described is arranged as follows: 'In tube 5I the controlv grid is 52, the cathode 53, the heater 54,

Vtioxneter 59 which is connected across battery 50 whosenegative terminal is grounded at 52.l A

byl-pass condenser .5 I` may be used between cathv ode 5 3 and ground 52.

The negative terminal' of 1a 'batteryrssfiso grounded at 52 and its positive terminal is connected to non-inductiveresistance 55. Resistance 55 in tufnconnects to anode 55 and also t coupling condenser 66. The output terminals are represented by 61 and 58. A by-pass condenser 59 shunts the B battery 63. A potentiometer 15 is connected across all or .a suitable portionxof battery 63. The slider of potentiometer 10 is connected to screen grid55from whichk by-pass condenser 1i is connected to' ground S2. Again it will be noted that the second tube andcircuit of Figure V is not unlike the arrangement of Figure I.

The operation of each of the tubes 3i and 5|,j

and their associated` circuits corresponds substantially with the operation andtheory ot Figure I. Substantially the same voltages, resistances, and adjustments may be employed for,

`Figui-eV as already described for Figure I. Since the voltage relation of tube 3i is substantially 180 degrees out of phase with tube 5i.. it follows'tha't as grid oi' tube-3i.Y is made less negative and is 40 'approaching its critical angle A, the grid of tube 5| is going further from its critical angle 1B.

Likewise when the plate current of tube 3l starts to vary, the plate current of tube. 5I varies inthe opposite direction or may become iixed. In the practical operation of Figure V, .I set the bias voltage on the grid 32 to zero. `Next the bias voltage on-.grid- 52 is adjusted by means of 59 until the grid is on a desired portion of the slope. Thenfor a given variable voltage applied between .31 and 38,- I find experimentally the optimum value oi grid voltage on 32 by means .of 39. y

If the various potentials of Figure V are carefully chosen, a condition is reached when the-in- Q put voltage across -31--33 may be varied `between 1 wide limits and the, output currents across 51-,55

will be constant* Departing `slightly from this condition by, suitably biasing grids 32 and152, I can arrange the gate action described above.

60 The bias of the grid of the second tube determines the size ofthe gate, and the operating position o n the characteristic curve.` of the gate is deterq mined by the bias onthe grid of the'flrst tube. same adjustment may be made by'xi'ngthe 5 biases on the grids32 and 52 and shifting the location oi' the angles Aand B by varying the potentials on screen grids' 35 and 55.-.

` The characteristiccurve of the gate.v combination or my'invention can b e observed in vFigure t o VL 'In the Ease oran applied alternating cur- A and B, the plate current or the output tube -varles uniformly.

angles A and B, `no change is .observed in the plate current of theoutput tube. In Figure VI Beyond the. voltage limits of for thesake of simplicity I have-shown angleA- 5 as occurring in the second tube but it should be understood that angle A actually occurs in the iirst tube and `is reilected in the anode current of the second tube.

10' the screen grid'55 and the anode 55. The control grid 52 is connected to .the anode 55 or tube Ii..

'I'he cathode 52 connects to the slider otpoten- 1f the grid of .the input tube is, biased to point 1o `X, all impressed .voltages onthe grid, up to the limits of Y and Z, `will be faithfully reproduced Vby corresponding anode currentl changes in the t s econdtube. All voltages von the rst grid in ex-i cessof Y and Z will be cut-oi by the sharp 15 limits of the critical angles AA and B. 'I'he gate may be made extremely small so that the potentials beyond a very small fraction of a volt are cut ofi. Likewise suitable grid bias voltages may -be employedto pass' considerably higher voltages. 2o l In practice the circuit of Figure V may be used -A in radiofrequency receptionof telephonic or telegraphic signals Without distortion, and excessive voltages,- representing undesirable signals, will be cut `oif orjlimited by the gate action described. 25

In the case of telephonic radio signals the gate is made sufiiciently large so that the modulations of the carrier frequency are not cut oi but all impulses in excess of the gate size are cut oft' as shown in Figure VI. 'Thearranjgement 'may 30 'also be employed in the elimination of one or both sidebands in modulated continuous wave transmission or reception. Further applications of the' arrangementwill be found in my co-pending application, Serial No. 6,015, filed February 11th,35 1935,?-entitled Radio circuits for static limitation. r

' Iclaim:

. l. A `i:hern:ti 'onic tube "circuit comprising, a thermionic tube having cathode, anode and con- 40 trol electrodes, an external anode circuit com-- prisin'g an impedance, means for limiting ,the flow of cur-rent `in lsaid anode circuit, and other 'ineans for decreasing the flow of current in saidanodecirc'uit. so that an abruptlchange is ef- 45 iected in the anode current. said abrupt change being'characterized by the junction of two angularly disposed lines within a radius representedby less than one-tenth volt on "the characteristic in- .putvoltage-output current curve of said tube and 5 0. circuit.

. 2. A thermionicI tubeV circuit comprising, a

, thermioni'c tube having cathode, anode and' control electrodes, an external anode circuit com# prising a resistance; means f or limiting the iiow 55 :of current in said anode circuit, and other means for decreasing the'flow of current in saidanode i thermionio tube'having cathode, anode and co'ntrol electrodes, an external anode circuit comvprijsing a resistance, means for determining the normal slope of: the gridv voltage anode current c teristic of said tube, and means for abrupt- 1y altering saidV 'slope to eiilect a characteristic of constantjcurrentinsaidanodecircuit. the change 'fromsaid slope to'said constant currentbeing 70A represented bya radius occupying not'more than one-tenth volt on 'the gridvoltage-anode current characteristic of said tube and ,'cir'cuit.

4.`A'thermionic tube circuit. comprising,v a

.vaclnimftube having cathode, anode, and control electrodes; an anode circuit comprising said cathode, saidl anode, a non-inductive resistance, and a source of potential; a control electrode for limiting the maximum ow of current in said anode circuit; and a second control electrode for varying said anode current below but not above said maximum current ow, the junction of the characteristic curve. of said varying andsaid maximum current characteristics being effected by not more than one-tenth volt change of input voltage.

5. A thermionic tube circuit comprising, a thermionic tube having cathode, anode, and grid electrodes;- a source of anode potential; means for limiting said anode potential to a value insuring constant current in the anode circuit; and means for impressing a constant potential on said grid electrode so that varying potentials applied to said grid electrode will decrease but not increase said constant current, the junction of the characteristic curve representing said decreasing and said constant anode current being represented by a change of less than one-tenth volt grid potential.

6. A thermionic tube circuit comprising, a screen grid thermionic tube having cathode, anode, screen, and control electrodes; a source of anode and screen potentials; means for establishing said screen potential at a value insuring current saturation in the anode circuit; means for applying a constant potential to said grid electrode so that varying potentials applied to said grid electrode will decrease but not increase said saturation current, and establish a sharp angular junction between said decreasing currents and said saturation currents represented by a grid potential change of less than one-tenth volt.

7. A thermionic tube circuit comprising, a thermionic tube having a uni-potential cathode, anode, and grid electrodes; a, source of anode potential; means for limiting said anode potential to a value insuring constant current in the anode circuit; and means for impressing a constant potential on said` grid electrode so that varying potentials applied to said grid electrode will decrease but not increase said constant current, the junction of the characteristic curve representing said decreasing and said constant anode current being represented by a change of less ythan onevtenth Volt grid potential.

8. A thermionic tube' circuit comprising, a

`screen grid thermionic tube having auni-potential cathode,v anode, screen, and control electrodes; a source of anode and screen potentials;

means for establishing said screen potential ata value insuring constant current in theanode circuit; means for applying a constant potential to said grid electrode, whereby varying potentials applied to said grid electrode will decrease but not increase said constant current, and having a characteristic input voltage-output current graph in `which the junction of said vconstant current` and said decreasingcurrent is represented by ,a

radius equivalent to less thanone-tenth volt input voltage change. e

9. A ythermionic tube circuit for detecting radio frequency currents comprising, ay thermionic tube having cathode,` anode, and grid electrodes; a high resistance external (to said tube vconnected lto said anode for limiting the anode potential to a value normally insuring constant current in the anode circuit; means for normally biasing said grid electrode to a potential not effecting said constant current; means for coupling a signal indicating instrument to said 'radius equivalent to less than one-tenth volt input voltage change.

10. A thermionic tube circuit for detecting radio frequency currents comprising, a thermionic tube having cathode,'anode, screen, and grid electrodes; a-high resistance external to said tube connected between said anode and cathode; means for impressing a constant potential on said screen electrode to normally insure constant current inthe anode circuit; means for normally biasing said grid electrode, to a potential not efiecting said constant current; means for coupling a signal indicating instrument to said anode circuit, and having a characteristic input voltageoutput current graph in which the junction of said constant current and the portion approaching said constant current is represented by a radius equivalent 'to substantially less than onetenth volt input voltage change.

l1. A-thermionic tube circuit comprising, two thermionic tubes connected in series each having cathode, anode, and control electrodes; means for limiting the anode potential of each of said tubes to values normally insuring constant current in the anode circuit, means for impressing a constant potential on each of sai-d control electrodes, means for connecting the output of the iirst ftube to the input of the second, and represented by an operating graph in which the constant anode currents are represented by two substantially parallel lines, each terminatingin a common sloping connecting line in which said straight parallel lines and said sloping line have junctions which are equivalent to a radius of less than one-tenth volt of input potential.

12. A thermionic tube circuit comprising, serially connected thermionic tubes each having cathode, anode and control electrodes; a connection from the anode of the first tube to the control electrode of the second tube; means normally determining constant currents in the anode circuits of each tube; means for applying constant potentials between cathode and control electrodes of each ofsaid tubes, and having an operating characteristic represented by a graph having substantially parallel lines for the constant anode currents, each terminating in a common sloping. line, said terminations being equivalent to a radius of substantially less than onetenth volt input potential.

13. A thermionic tube circuit comprising, serially connected thermionic tubes each having uni-potential cathode, anode, and control electrodes; a connection from the anode of the first tubeito the control electrode of the second tube; means determining the saturation of the currents in the anode circuit of each tube; -means for applying constant potentials between cathlode and control electrodes of each of said tubes, and represented by an input voltage-output current operating graph having substantially parallel lines representing said saturation currents andterminating in a common sloping line, each termination being equivalent to an input voltage change of less than one-tenth volt.

14. A thermionic tube circuit comprising, a pair of serially connected vacuum'tubes each having cathode, grid, screen, and anode electrodes; a connection from the anode of the rst tube to the grid of the second tube; connections for applying a constant potential on the -screen grid of `each'tube. for determining constant currents in 75 V l 2,095,361. the anode circuits in said pair ofhtubes, biasing f voltages for the grids onf each of said tubes, and

further characterized by an` input voltage output current curve which has a constant current portion, a variable current portion and a second constant currentuportion, the junctions of said portions being effected by an input potentialchange of less than one-tenth volt.

15. A thermionic tube circuit comprising, a pairI of serially connected vacuum tubes each having a uni-potential cathode, grid, screen, and anode electrodes; a connection from the anode of the` first tube tothe grid of the second tube; connection for applying a constant potential on the screen grid of each tube, for determining the saturation of the currents in the anode circuits in y said pair of tubes; biasing voltages for the grids of each of said tubes so that variable voltages applied to the grid of the first tube produce changes in the anode current of the second tube lnot exceeding sharply defined limits and characterized by an input voltage-output current" graph having a sloping portion terminating in two non-sloping at portions, said terminations having a radius of substantially less than onetenth volt of input potential.

16. A thermionic tube circuit comprising, two serially connected thermionic tubes each ,having cathode, anode, and control electrodes; a high resistance connected to the anode of the iirst tube; a second high resistance 'connected to the anode ofthe second tube; .a connection from .the anode of the rst tube to the control electrode of the second tube; sources of anode potentials for each of said tubes normally insuring currents in each anode circuit; biasing voltages for the control grids so that varying voltages impressed o'n the control electrode of the iirst tube produce vchanges in anode current of the second tube within the limits determined by said anode constant currents.

and characterized by Vfari input voltage-output current graph having a variable voltage variable current portion terminating in two constant current variable voltage portions, said terminations having a radius of substantially less than one` tenth volt of input potential.

'17.' A device of the character described, in-

cluding `serially connected thermionic tubes, va connection from the anode of the first tube to the grid of the second tube,'meansl for. limiting the maximum anode currents in each of said tubes v to predetermined constant currents and means which'limit'the output currents of the second tube to sharply dened minimum yand maximum values so that the input voltage output current characteristic may be represented by a graph having a sloping portion terminating in nonsloping portions equivalent to said minimum and maximum values, the `angular intersection of said portions having radii4 equal to less than onetenth volt of input potential change.

18. vA device of the character described, includl lrepresented by a graph having a sloping portion terminating in non-sloping portions equivalent to said minimum vand maximum values, the angular intersection of said portions having radii equal to less than one-tenth volt of input lpotential change.

' D AVID G. MCCAA.- 

